The interaction between an exoskeleton and its human user is complex, and needs to conform to various requirements related to safety, comfort and adaptability. It is however impractical to test a large number of prototype variations against a large number of user variations, especially in the initial design and testing phases. Model based methods can help at this design stage by providing a virtual testbed. In this study, we develop a MATLAB-based toolbox that can simulate the interaction between a human model and a lower limb exoskeleton during the sit to stand motion. We present results for different scales of human users as well as variation in levels of exoskeleton assistance. Our results show that large reductions in human joint torques upto 57Nm are possible, while also transmitting large forces upto 300N to the human model. Additionally, by varying the human body sizes by 15% we found that the interaction forces also changed by as much as 29.2%. Therefore, careful consideration of the human user and its limitation should be made in the exoskeleton design and concept phases.